This invention relates to an apparatus for locating a plurality of articles, more particularly but not exclusively the articles are located for laser marking; markable articles are used for example in combinatorial chemistry.
Combinatorial chemistry is a technique whereby very many different chemical compounds are produced by multiple chemical reactions. Articles (commonly known as beads) of the size approximately of a tenth of a millimeter (mm) in diameter are typically used in such reactions. Molecules can be attached to the articles by way of chemical bonds sometimes called xe2x80x98hooksxe2x80x99.
To form a library of chemical reagents, it is usual to start with a large number of articles. In order to illustrate a combinatorial process, it is useful to consider the following example. Articles are divided into 3 groups. A different reagent (A, B or C) is then be added to each group of articles. There are now 3 types of molecules attached to the articles: articles in group 1 have molecules of reagent A attached to them, articles in group 2 have molecules of reagent B, and articles in group 3 have molecules of reagent C. Next, the 3 groups of articles are pooled, mixed up, and again split up into 3 groups. Three more separate reactions are then carried out. This results in a combination of 3 reactions in the first stage of the process, and 3 reactions in the second stage, producing 9 different species of molecule. If the groups of articles are again pooled, mixed up, and split up into 3 further groups, 27 different compounds are generated.
The library of compounds created by the above steps is known as a 3xc3x973xc3x973 library. Nine individual (3+3+3) reactions have been carried out, and 27 (3xc3x973xc3x973) different compounds have been generated. Other sizes of library may be created by varying the number of reaction or synthesis stages, and by varying the number of groups of articles.
Laser marking is used as a method of tracking the articles in the above mentioned combinatorial synthesis. In previous methods of laser marking, each article was placed on a flat surface. A laser beam was applied to an upper surface of the article, which was typically spherical or close to spherical, such as an oblate sphere. This method relied on the laser beam focussing on each article to be ablated so as to achieve marking. Since the spherical articles were often of varying sizes, considerable effort and time was expended in focusing on each bead before leaser marking could occur. Accordingly the method produced labelled articles but did not result in a very high throughput.
United Kingdom Patent Application BG 2 306 484 (University of Hertfordshire) discloses a machine readable code for use in combinational chemistry techniques. The code may include pits, holes, hollows or grooves.
It is an object of the present invention to alleviate the aforementioned problem.
According to the present invention there is provided an apparatus for locating a plurality of articles comprising: a support plate having apertures extending from a first surface of the plate to a second surface of the plate, the aperture(s) being dimensioned to receive said article(s) from the first surface so that at least a portion of the article is presented to be viewed from the second surface of the plate; characterised in that the diameter of each aperture is less than 200 xcexcm.
By selecting the diameter of the aperture especially of the aperture of a first surface and thickness of the support plate, the extent to which spherical articles protrude through the first surface of the support plate is significantly less than the variation in diameter of the articles to be marked. By using this apparatus, spherical articles can be marked more efficiently without the laser having to re-focus on individual spherical articles. The throughput for marking spherical articles is therefore greatly improved.
The protrusion of the spherical article (or bead), through the aperture on the first surface, can be represented by the formula:
r(1xe2x88x92sin(xcex8)).
where r is the article (or bead) radius and where r.cos (xcex8)=xcfx89/2 where xcfx89 is the diameter of the aperture and xcex8 is the angle enclosed between an intersecting radius and line defining a segment, the point which the sphere touches an edge of an aperture.
There is a simple relationship between the article protrusion (d), the article radius (r), and the diameter of the aperture(xcfx89). This is illustrated diagrammatically in FIG. 4.
xcex1=sidewall angle
xcex1 greater than xcex8 very bad, bead sits on wall
xcex1=xcex8 apparantly ideal
xcex1 greater than xcex8 line contact assured
say xcex1=critical contact for 10% undersize bead
for standard bead we have r cos xcex8=xcfx89/2.
for undersize bead 0xe2x88x92g r cos xcex1=xcfx89/2
0.9 r cos xcex1=r cos xcex8  =      α    =                  cos                  -          1                    ⁢              (                              cos            ⁢                          xe2x80x83                        ⁢            θ                    0.9                )            
Generally if beads are from
Arnxe2x89xa6rxe2x89xa6bra (axe2x89xa61, bxe2x89xa71)
  α  ≤            cos              -        1              ⁢          (                        cos          ⁢                      xe2x80x83                    ⁢          θ                a            )      
For example: the diameters of articles may vary by xc2x110%. The apertures in the support plate may vary by 5%; through sophisticated fabrication techniques render variation plate apertures negligible. Assuming the articles have a nominal diameter of 300 xcexcm and the apertures in the support plate are of 100 xcexcm diameter; if a flat surface is used to support the articles, the change in article height would be 30 xcexcm, i.e. xc2x110%, for a xc2x110% diameter range. However, when an article is located in an aperture, the variation in the protrusion of the article is only xc2x12 xcexcm, i.e.  less than 1%.
The apertures in the first surface are preferably dimensioned so that substantially similar projections of spherical articles protrude through the aperture. The apertures optionally comprise side walls which are substantially tapered. The tapering of the side walls restricts the movement of the spherical articles to be marked or ablated.
In an alternative embodiment, in use, spherical articles are disposed between the support plate and a deformable layer. The deformable layer is positioned so as to restrict the movement of the articles which are subsequently ablated so as to achieve marking.
Most preferably a force is applied to the deformable layer further restricting movement of the spherical articles and maintaining the articles uniformly in the apertures. The force may be applied to the upper surface of the layer.
The support plate is preferably planar. The diameter of apertures are preferably less than 100 xcexcm and most preferably less than 50 xcexcm. By limiting the diameter of the apertures within the support plate variable sized spherical articles can be received within the apertures so that at least a portion of the article is visible from the other side of the support plate.
In a further alternative embodiment there is provided an apparatus for locating a plurality of spherical articles which includes the features of the first embodiment and further comprises a placement means for placing articles onto the support surface, the surface being displaceable with respect to the placement means and a discharging means for releasing or ejecting the articles at a predetermined instant.
The discharging means preferably comprises an air jet so that articles can be removed efficiently from the support plate after the laser has marked the articles. Other means of discharging the spherical articles from the support plate may include any of: a vacuum, magnetic, electrostatic device or a mechanical grabber.
In a yet further embodiment there is provided an apparatus for locating a plurality of articles including the features of the first mentioned embodiment and further comprising: a conveyor for displacing the surface with respect to the placement means. Preferably a collector is also provided to recover the articles when they have been removed from the support plate.